The present invention relates to a planar filter suitable for use, for example, in microwave bands including millimeter wave bands, and more particularly to a planar filter preferable for use in high-frequency radio communication devices such as millimeter wave communication devices using a frequency of 30 GHz or more, as well as to a semiconductor device and a radio unit having the planar filter.
Conventionally, there have been planar filters that use microstrip resonators. A design method thereof is described in, e.g., a literature “Basics and Applications of Microwave Circuits” by Yoshihiro Konishi, pages 369-373, published by Sogo Denshi Publishing, Aug. 20, 1990).
FIGS. 6A and 6B show one example of conventional planar filters. FIG. 6A is a plan view and FIG. 6B is a cross sectional view taken along the line D-D′ in FIG. 6A. The planar filter is structured such that an input line 1, an output line 2, a resonator 3, a resonator 4 and a resonator 5 are formed on a dielectric substrate 10 having a grounding conductor 11 on the back face. Each of the resonator 3, the resonator 4 and the resonator 5 has a line length that is half an equivalent wavelength of a passband center frequency.
As shown in FIG. 6A, a part of the input line 1 and a part of the resonator 3 are in parallel proximity to each other with a constant gap therebetween to thereby establish electromagnetic coupling. Also, a part of the resonator 3 and a part of the resonator 4 are in parallel proximity to each other with a constant gap therebetween so as to achieve electromagnetic coupling. In a similar manner, the resonator 4 and the resonator 5, as well as the resonator 5 and the output line 2 are in parallel proximity to each other with a constant gap therebetween for electromagnetic coupling, respectively. By appropriately disposing the resonators 3 to 5 and the input/output transmission lines 1, 2 to optimize the degree of coupling, a desired bandwidth can be achieved. The shown planar filter has thee resonators 3, 4, and 5. It is to be noted that while a larger number of resonators can increase attenuation outside the band, it also increases loss in the pass band and the area to be occupied by the filter.
The shape and arrangement of the resonators in the conventional planar filter shown in FIG. 6 has following problems. If the resonators are arrayed in a longitudinal direction, the size of the planar filter is increased. Particularly in the case where the planar filter is integrated on an IC chip for reducing a loss in a connection section between the planar filter and other high-frequency integrated circuits, the conventional resonator layout deteriorates space efficiency of the IC chip and increases dead space not available for other circuits, with the result that the size of the IC chip and the unit cost of the chip are increased.